The wife should have been shown in the film in some capacity. It's hard to take a villain too seriously when his motives are complete hearsay. The issue was only addressed by one throw-away line that dismissed the issue and shooed it off screen in hopes no one would notice.
There needs to be some kind of "proof" for the audience.
Audiences so not need a visual reference for personal stories, as the person telling it is considered a believeable source. Only screenwriters thinkinf audences all suffer from the videogame mentality--where everything is visually spelled out--resort such uneccessary scenes, and it becomes less about character building than setpieces.
One great example where no visual reference was needed was in Jaws
; we did not need a visual flashback to Quint's experience with shark attacks after the sinking of the Indianapolis
to understand his determination to kill THE
shark, or why he said he would never wear a lifejacket again. We got it. Similarly, we did not need Marla on screen (in flashback to her death, or anything else), to accept why Khan would want to kill the man he held (ultimately) responsible for exposing her to the creatures of that world.
I seriously doubt anyone in 1982 (or beyond) questioned Khan's motivations.
They could've used someone else. No one would notice.
It is called respect for the actress who brought the character to life.